In satellitic broadcast wave reception or satellite communication, generally, there has been employed a communication method for transmitting different sorts of information with different polarized electromagnetic waves used in respect of waves of the same frequency for the purpose of effective utilization of the frequency. In the case of, for example, circularly polarized electromagnetic waves, it is possible to transmit the different sorts of information with a left handed polarized electromagnetic wave and a right handed polarized electromagnetic wave or, in the case of linearly polarized electromagnetic waves, the transmission of the different sorts of information can be made with a horizontally polarized electromagnetic wave and a vertically polarized electromagnetic wave. Here, it becomes necessary for receiving the two different polarized electromagnetic waves to render a receiving system to be of a simultaneous-dual type, employing two single polarized electromagnetic wave receiving LNBs, or to be of a switchable-dual type with a polarizer means additionally provided to a single, polarized electromagnetic wave receiving LNB.
However, there has arisen a problem that the on current use of the two single polarized electromagnetic wave receiving LNBs or the provision of the polarizer means with respect to the single polarized electromagnetic wave receiving LNB renders the external formation of the LNB to be excessively larger and the entire weight of the device to also be larger. This problem has been particularly remarkable in the case where an LNB is applied to a flat antenna, where compactness, thinness and lightness are critical.
In the dual polarized electromagnetic wave receiving LNB, further, the frequency conversion is executed preferably with a locally oscillated signal from a local oscillator utilized for lowering the frequency of the received signals, and the received signals subjected to the frequency conversion for the respective polarized electromagnetic waves are alternatively provided as the output. In this arrangement of employing the local oscillator for the frequency conversion, on the other hand, it is unavoidable that the locally oscillated signals from the local oscillator will partly leak to the input side and to the output side as well. Taking into account any external influence thereof, such partial leakage of the locally oscillated signals should preferably be reduced as much as possible, and the leakage signal to the input side in particular is required to be sufficiently restrained since such signal is caused to be radiated through the antenna.
Generally, in the dual polarized electromagnetic wave receiving LNB of the type referred to, there are employed microstrip lines in an interior circuit, and it is known that the locally oscillated signals leak through the microstrip lines to the input side and also propagate through the interior space of the LNB. In the interior circuit, therefore, metal plates are provided between respective constituent elements as partitions for dividing the interior space into a plurality of regions mutually separated so as to restrain the spatial propagation of the locally oscillated signals. In an event where respective means for processing the polarized wave signals received include a radio frequency amplifier, it has been ascertained that there arises a variation in the signal transmission characteristics between occasions where each of the radio frequency amplifiers does and does not operate to alternately provide the output, and the problem remains that the received signals are caused to partly leak to the input side.
There have been disclosed converters to which the two different polarized electromagnetic waves are input, in Japanese Utility Model Laid-Open Publication No. 62-181007 of R. Sato; Japanese Utility Model Laid-Open Publication No. 1-133801 of K. Kajita; Japanese Patent Laid-Open Publication No. 2-63201, corresponding to British Patent Applications Nos. 88 16273.0 and 89 01278.5 of S. J. Flin et al.; Japanese Patent Laid-Open Publication No. 2-223201, corresponding to British Patent Application No. 88 16276.3 of K. R. Haward; Japanese Utility Model Laid-Open Publication No. 3-36243 of R. Koiso; and Japanese Patent Laid-Open Publication No. 3-228401. In any one of these prior art references, however, there has been disclosed no converter which satisfies the demands of compactness, thinness and lightness.